Millimeters are the native unit of engineering drawings, metric fasteners, camera and phone specs, and 3D printing. Inches — especially fractions of an inch — are what American tape measures, drill indexes and socket sets are marked in. This converter gives both the precise decimal inch and the nearest fraction you can actually find in a toolbox, along with how far off that fraction is.
How to use the mm to inches converter
- Enter the length in Millimeters (mm).
- Read the decimal inches, then the nearest 1/16 inch and nearest 1/32 inch. Each fraction shows its error, marked “over” or “under”.
- The chart lists common metric sizes from 0.5 mm to 300 mm with the nearest sixteenth.
The mm to inches formula
The 25.4 is exact — one inch has been defined as 25.4 mm since 1959. To get a fraction, multiply the decimal part of the answer by the denominator you want (16 for sixteenths) and round:
Worked example: is a 10 mm socket the same as 3/8 inch?
Step 1. 10 ÷ 25.4 = 0.3937 in.
Step 2. 0.3937 × 16 = 6.30, which rounds to 6/16 = 3/8 in (0.375 in).
Step 3. The difference is 0.3937 − 0.375 = 0.0187 in. A 3/8 socket is too small to fit a 10 mm nut; the closer 1/32 fraction is 13/32 in (0.40625 in), which is too loose.
Metric vs SAE wrench sizes
Mixed toolboxes are common in garages, so it helps to know which sizes overlap. The tighter the gap, the safer the swap:
| Metric | Decimal inch | Closest SAE | Difference |
|---|---|---|---|
| 8 mm | 0.315 | 5/16 (0.3125) | 0.0025 in |
| 10 mm | 0.394 | 3/8 (0.375) | 0.019 in |
| 13 mm | 0.512 | 1/2 (0.500) | 0.012 in |
| 16 mm | 0.630 | 5/8 (0.625) | 0.005 in |
| 19 mm | 0.748 | 3/4 (0.750) | 0.002 in |
| 22 mm | 0.866 | 7/8 (0.875) | 0.009 in |
A difference above roughly 0.01 inch is enough to round off a tight bolt, so use the correct system whenever the fastener matters.
Lumber and sheet goods
North American plywood sold as “3/4 inch” is usually 23/32 in thick, which is 18.26 mm, while European sheets are often a true 18 mm (0.709 in). The two are close enough to mix in cabinet carcasses, but a dado cut for one will be loose or tight for the other. Always measure the actual sheet with calipers before cutting joinery.
Rainfall and small measurements
Weather services outside the US report precipitation in millimeters. One inch of rain is 25.4 mm, so a 10 mm downpour is 0.39 in and a 50 mm storm is nearly 2 inches. For thin materials — sheet metal, guitar strings, filament — keep at least four decimals, or switch to thousandths of an inch (“thou” or “mil”): 0.1 mm is 3.94 thou.
When to use centimeters instead
If you are converting body measurements, furniture or anything over about 30 cm, the cm to inches converter is more convenient, and it shows feet and inches. To add or subtract fractional inch measurements after converting, use the feet and inches calculator.
Frequently asked questions
How many millimeters are in an inch?
Exactly 25.4 mm. So to convert millimeters to inches, divide by 25.4; 1 mm is 0.03937 inches.
How do I convert mm to a fraction of an inch?
Divide by 25.4 to get decimal inches, multiply the decimal part by 16 (or 32, or 64) and round to the nearest whole number. That number over 16 is your fraction. For 10 mm: 0.3937 × 16 = 6.3, which rounds to 6/16 = 3/8 in.
Can I use a 19 mm wrench on a 3/4 inch bolt?
Usually yes. 19 mm is 0.748 in, only 0.002 in smaller than 3/4 in, so the fit is snug but workable. Pairs that differ more, like 10 mm and 3/8 in (0.019 in apart), will slip and round off the fastener.
What is 1/4 inch in millimeters?
Exactly 6.35 mm, because 0.25 × 25.4 = 6.35.